Here is a very comprehensive list of tips for improving your email deliverability when sending marketing email.

It is a tricky balancing act between sending spammy emails and actually providing value to get people to open your messages. A very nice trick that I would add is to send them something useful — at no cost — in an email at least once immediately after they subscribe or maybe even from time to time after. For example, offer a free report, a free application, a free piece of software. Anything they can learn from or use and expect from you is going to add a lot of value. People will actually look forward to your emails and will not unsubscribe.

Find out more at: http://gaveltechnologies.com/blog/tips-for-improving-email-deliverability